Compensated pre-heat coil



April 1939- H. R. CRAGO COMPENSATED PREHEAT COIEL Fi-led. March 5, 1956Inventor: Harvy wcragc, ,1/ (A His Attorney.

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v ments.

PatcntadApr. '25, 1939 es'rsn'r orFicE success COMYENSATED FEE-MAT con.

Harry R. Lrago, fialdwell, N. 3., designer to General Electric Gonipany,s cormrstion or New York Application March 5, less, seen! no. euros s@lalms. lei. some) My invention relates to temperature control andprovides an improved preheat thermostatic modulating control that ispeculiarly suited for use with modulating heat transfer systems butwhich "may be used with other types as Well.

The present invention is advnntsgeously used in heating service where athermosteticnlly graduated or modulated variation in the rate oi heattransfer is desired, but is npplicalole equally well to allsystemscontrolled by thermostatic systems using a. local pro-heat at thethermostst to obtain a more even temperature in the specs to be heated.

In the copending application oi Marcus E. Flene, Serial No. 25,691,filed June 8, i935, and assigned to the assignee of the presentapplication, there is disclosed c. modulating vnpor heat transfer systemin which the rate of heat transfer to a radiator for heating air orliquid may be varied smoothly and quickly between wide limits to meetwidely varying heating recluse- In that system a fiuldvaporlzing andcondensing surface is connected to receive heet from. a suitable sourcesuch es a steam supply main or the like and is provided with a liquidcontrol chamber which is preferably out of heat transler relation withthe vapor system and has a separate pilot heater therefor. Thevaporizing surface receives heat from the source and the condensingsurface releases heat to the air, water or other medium which is to beheated. The

liquid control chamber is so arranged and connected that when theseparate pilot heater is imactive, substantially all of the vapor in theheat transfer system condenses and accumulates as liquid in the liquidcontrol chamber. Under these conditions substantially no transfer ofheat from the supply source can occur.

Upon operation of the pilot heater the temperature within the liquidevaporating chamber is raised and the liquid is expelled therefrom intothe heat transfer system. The expelled liquid efiects transfer of heatfrom the heat source through the vaporizing-surface to the condensingsurface. Due to the establishment of thermal and hydrodynamicequilibrium in the system the liquid expelled from the control chamberis always automatically properly proportioned in amount to maintain thearea of wetted surfece of the vaporizing surface just sufilcient toefiect the transfer of heat from the source at the rate required toequalizethe temperature of the condenser surface with that of thecontrol chamber, the limit of course being the temperature of the heatsource. This conditionobtains irrespective cruise-e in the vaporizer.Conversely, should the temperature oi the condenser surface rise upondecreased dissipation of heat therefrom, the in creased pressure in thevapor system becomes efiectlve to lorcethe return of fluid to thecontrol chamber until the wetted surface of the veporlrer is decreasedto the value providing the required rate of heattrnnsfer to maintain thecondenser surface ternpemture the same as the control chambertemperature.

Selective vnriation of control chamber ternperature will result,therefore, in corresponding changes in the temperature of the condensersurface. By selectively varying the heat input or" u smell cepncitypilot heater the temperature oi the control chamber readily may bevaried between limits as wide as desired to efiect a. correspondingvariation in the temperature oi the condenser surface as well as acorresponding variation in the rate 02 heat transfer from the heatsource to the condenser.

In the application mentioned the pilot heater is illustrated as anelectrical heating unit of relatively smell capacity, selectivelyenergized by a thermal responsive means, such as a thermostat,positioned-in the space the temperature of which. is to no controlled.The capacity of the heater is such as to expel all the liquid in 15 orso ndnutes. Immediately upon energlnation n portion of the fluid isexpelled, and the longer the heater is energized the more fluid will beexpelled with the result that a greater area or vaporizing ill surfaceis wetted and the temperatures of the w control chamber and condensersurface equalize. After the temperature of the space reaches the desiredvalue the thermostat deenergizes the heater allowing the temperature ofthe control chamber and the condenser surface to fall. The

by providing the 2 4 control thermostat with means for locally heatingit, as by a pre-heat coil. Application of preheat to the thermostatcauses the latter. to operate at relatively frequent intervals,dependent 'upon the amount of pre-heat, intermittently energizing theheater. Due to this intermittent heating and the heat storage capacityof the pilot heater the temperature of the liquid is maintained at anaverage value varying but slightly from that predetermined value atwhich heat is transferred to the heated space continuously and at a ratejust suflicient to overcome losses due to dissipation.

This type of modulated control resulted in certain disadvantages due tosubcalibration of the thermostat when the pilot heater was energized forgreater lengths of the time in response to increased demands for heat.In other words, when temperature conditions are such as to rel quire thethermostat to energize the heater for "greater lengths of time, thepreheat coil is energized over a longer period to apply more preheat.Furthermore, to increase the frequency of operation of the thermostat toachieve better results through a more critical modulation, the preheatcoil must have a greater capacity. Thus, in cold weather, with arelatively large capacity preheat coil, the locally applied heatactuates the thermostat to its off position at a temperature lower thanthat which it is calibrated at, with the result that lower temperaturesare maintained within the space during cold weather.

The above described *subcalibrationoccurs not only in the arrangementdescribed but in all heating systemswherein a preheat coil is associatedwith the control thermostat. It is'obvlous that when the thermostat isin a position wherein it calls for heat and the pre-heat coil isenergized for a greater percentage of time the re- It is the principalobject of my invention to provide an improved temperature control systemin which such subcalibration is avoided and heat is transferred atvariable rates to maintain a predetermined temperature condition morenearly constant under all operating conditions.

-It is a further object of my invention to provide improved means forautomatically varying the effect of local heating means associated witha thermostat in response to increased demands upon the heat exchangercontrolled thereby.

A still further object of my invention is to provide means fordecreasing the efiect of the local heating means on the thermalresponsive means during periods when the latter is in a position callingfor heat during a relatively greater percentage of the time.

Briefly, this is accomplished by connecting in I series with thepro-heat coil of the thermostat an auxiliary resistor having a hightemperature coefflcient of resistance, preferably surrounded by somematerial as a metal to give it a high thermal capacity and suitablylocated at some point remote from the thermal responsive element orthermally insulated from the latter. During mild weather when thecontrol thermostat calls for heat only a relatively small percentage oftime the resistor remains at a comparatively low temperature, at whichit does not materially affect the flow of current through the pro-heatcoil. However during cold weather, with comparatively longer and morefrequent calls for heat by the thermostat the resistor is heated to ahigh temperature. -Its resistance increases and consequired. Forinstance, it could be applied to the control of a valve regulating theflow of a gaseous or liquid fuel to a burner, or the position of thedamper of a furnace as well as to all types of thermostatic controlwhere a source of local heat is used to modify the action of athermostat. My invention in its broader aspects is thus not limited toany particular type of heat transfer system.

- A more detailed understanding of the present invention may be securedfrom the' following description taken in comiection with theaccompanying drawing in which is illustrated a preferred embodiment ofthe invention.

The single figure of the accompanying drawing diagrammatically shows,partly in section, an air heating radiator unit with a fluid vaporizingand condensing heat transfer system therefor deriving heat from a steamchamber and the liquid control chamber electrically heated under thecontrol of a thermostatic switch provided with the usual pre-heat coiland a preferred form of auxiliary resistor.

In the illustrated embodiment there is provided a radiator 9 for heatingthe air in the room indicated by the dotted lines I 0. This radiator ispreferably formed of suitably pressed metal plates welded together toform a series of interconnected vapor condensing columns I l with openair circulation passages l2 therebetween to facilitate dissipation ofheat from the radiator to the air. The bottom wall of the radiator 9 ispreferably sloped so as to readily drain the condensed vapor into thecondensate receiving tube l3. Tube I3 is joined with the vaporizing tubel4 having one end thereof bent downward and extending into the steamchest I 5 in which live steam is maintained at all times. The steam issupplied from any suitable boiler or other source (not shown) by thesteam supply pipe l6 which also serves to return the condensed steam tothe source. A suitable heat insulating cover Il effectively prevents anydissipation of heat from the steam chest IS, the steam pipe l6, as wellas from the vaporizing tube l4 except through the'operation of the heattransfer system in a manner to be described hereinafter,

A closed liquid control chamber is located remotely from both thevaporizing surfaces of the tube l4 and the condensing of radiator 9 andhas a relatively small size tube 2| communicating between the bottomthereof and the bottom of the condensate receiving tube I3. This servesto minimize the transfer of any heat between the control chamber 20 andthe main heat transfer system consisting of tube l4 and radiator 9.

' An electrical heating unit 22 of the cartridge type is mounted insidethe tube 23 which is sealed into the liquid control chamber 20. Theelectrical heating unit 22 is of relatively small capacity and ,isenergized at low voltage-derived from the .secondary of the transformer24 to which it is connected by means of a switch 25 under the c ums:

control of thermal responsive means 2!. The thermal responsive meansconsists of a thermal responsive element 21 illustrated as being or thebimetallic type and adapted to close an energizing circuit for a relay28 which controls the position of switch 25 when the temperature withinthe space it! falls to a predetermined low limit. The energizing circuitextends from the secondary winding of transformer 24 to the right handcontact oi the thermal responsive means through a conductor 29, thebimetallic element in engagement with its right hand contact, connection36 leading to the relay 2B which in turn is connected to the otherterminal of the secondary winding through connection ill. The resultingenergizetion of relay 28 eflects upward movement of its associatedarmature 2t and closure of the energizing circuit to the heater 22 byclosure of switch 25. Simultaneously with the. closure of switch 25 aholding circuit for relay all is estah lished by closure switch 82 whichcuts out a portion of the circuit including the thermal element.

The energization of relay 2% also results in the flow of current througha circuit including connection pro-heat coil ti t, and an auxiliaryresistor 35 of material possessing a high temperature coefficient, suchas nickel or tungsten, suitahly mounted within a metallic hody Elli andconnected by means of electrical connection ill to electrical connection8%. It may he noted that the pro-heat coil and auxiliary resistor areenergized immediately upon the engagement by thermal responsive element27 with its associated right hand contact and that they remaiuenergizedupon the return of the holding circuit for relay 28 by closureof switch 32.

The metallic body St is preferably made oil some material which has ahigh thermal capacity such as brass or copper and the whole, in

cluding the auxiliary resistor 35 and the metal-- lic body may he placedat some point remote from the thermal responsive element El. The reasonfor this is that it is not desirable that the auxiliary resistor 35thermally affect the operatlon of bimetallic element Z'l.

. Inasmuch as it has been assumed that thermal responsive element 2'?moves to the right in response to a decrease in temperature, it isobvious that upon an increase in temperature it will move in theopposite direction. If the increase in temperature exceeds a desiredpredetermined limit the thermal responsive element will engage its leftcontact to short circuit the pre-heat coil and the auxiliary resistor aswell as the relay and thus effectively cleericrgize the latter.

Before describing in detail the operation of my system I shall brieflydescribe the construction of the heat transfer system and liquid controlchamber which form the basis of the above entitled Fiene application,Serial No. 25,691.

The vapor condensing radiator vaporizing tube i l, and the liquidcontrol chamber as well as the interconnecting tube l3 and iitherebetween are preferably all joined together and hermetically sealedby welding or brazing so that a. closed vapor-tight heat transfer systemis obtained. This entire system is then evacuated of substantially allnon-condensible gases through a suitable evacuating connection it.Thereafter a predetermined charge of suitable vaporizable liquid such aswater, alcohol or the like is introduced into the closed system. Theamount of this liquid charge is such as to insure that the entireeffective heat transfer system of the vapor lzlng tube within the steamchest it may he efiec= tively wetted under maximum heat transierconditions. Qrdinarily this condition may be obtained when'the volume ofthe liquid. charge is sufllcient to fill the effective portion of thevapor:

izing tube It within the steam chest I! substantially one-third full.Preferably the volume of the control chamber 20 is made somewhat largerthan necessary to contain the total amount of liquid with which thesystem is charged. This insures that substantially all of the liquid inthe system can be withdrawn into the control chamber 20 and therebypractically stop the transfer of heat from the steam chest l through thevaporizer M to the radiator 9 whenever required. Since the system isevacuated and charged with a vaporizable liquid, some extremely smallportion of the liquid will of course remain as vapor with the saturatedvapor pressure in the system corrwponding to the temperature of theremaining liquid. However, since the vapor remaining does not condense,practically no heat transfer can occur but conditions are suchthatadditional amounts of liquid will immediately vaporize whenever thetemperature oi the liquid in control chamber M is raised.

After the vapor system is exhausted and charged with the proper amountof fluid, any 1 small amount of non-condensibie gas which may happen toremain in the system will be forced by the movement of the vapor intothe upper portion of the radiator 9 without any serious interferencewith the operation of the vapor system. in order to reduce thenon-condensible gases in the sys= tern to a minimum, preferably theapparatus is baked or heated to a relatively high temperature during theexhausting process. After exhaustion and charging of the system throughthe charging connection fit is completed, this connection is pinched andsealed.

The wattage input of the electrical heating unit or pilot heater is madesuch that under ordinary ambient air conditions, heat will helll'lparted to the control chamber 26 at the proper rate to efiect thevaporization of a small portion of the liquid therein required togenerate a vapor pressure sufilcieut to expel substantially all of theliquid from the chamber it in a predetermined time interval such forexample, as 15 or 26 minutes.

The control cber til is so constructed that its cooling time correspondswith its heating time in order to provide the best conditions formodulated heat control operation by the thermostatic swltch 2%. In otherwords, the chamber is so prepared and designed that its rate of heatdissipation to its environment under normal con= clitions will reducethe temperature thereof. to substantially room temperature inapproximately a period of 15 to 20 minutes.

In operation when the temperature or the air in the enclosure or room itfalls below the predetermined value at which the thermostatic element itengages its right hand contact, relay Bil is energized by connectionacross the secondary oi the transformer through electrical connectionelement 2?, and electrical connection it. 'The resultis theestablishment or" a holding circuit for the relay by closure of switch32 and enertill As soon'as the temperature of the'liquid in chamber 2.is raised due to the heat input of the pilot heater 22 a small portionof the liquid is at once vaporized. The resultant vapor pressure inchamber 2| forces some liquid from the bottom of that chamber throughtube 2| into the condensate return tube I2. As soon as the level of tubeIt and radiator 9 retards or even stops momentarily the further supplyto control chamber 26 but as the temperature of the liquid in controlchamber 2|! continues to increase due to continued energization of pilotheater 22, an additional amount of liquid is vaporized therein and moreand more of the liquid is expelled into the vaporizing system.

As the rate of transfer of heat from the steam chest through thevaporizing system varies with the amount of liquid effective to wet thevaporizing surface, the heating action of the radiator 8 upon theambient air of the enclosure I0 is increased as more and more of theliquid becomes effective to wet the vaporizing surface. Consequently,the temperature of the ambient air to which the thermostat 28 isresponsive increases. The eil'ect of preheat coil 34 is to provide amodulated control by varying the time of response of the thermostaticswitch 26 to make it less than the heating and cooling time of controlchamber 20. Thus when the thermostatic switch 26 is made quicklyresponsive to an increase or decrease in the temperature of the ambientair in the enclosure II, the heat input of the pilot heater is startedand stopped at relatively frequent intervals. Because of thisintermittent heating action as well as temporary heat storage in chamberthe temperature of the liquid in chamber 20 is practically maintained atan average value which varies only slightly from the desiredpredetermined temperature value. This will result in maintaining thetemperature of g the radiator 9 at a corresponding average valueenergized for a greater percentage of time since they are energized andde-energized conjointly with the relay.

In order to obtain a more critical control, it has been found desirableto increase the frequency of operation of the thermostat 21. This hasbeen accomplished by increasing the heating eil'ect of pre-heat coil 34.By placing the auxiliary resistance 35 in series with pre-heat coil,subcalibration of the thermostat has been avoided to a large extentbecause of the fact that when the pre-heat coil is energized for agreater percentage of time the effective resistance of the auxiliaryresistance is increased and consequently, the flow of current throughthe pre-heat coil 34 is decreased. The resulting decrease in currentflow lessens the eifect of the pre-heat coil on the thermostat whenconditions aresuch as to' require more heating. In this manner, byvarying the time of response of the indoor thermostat in accordance withthe length of time that it calls for heat, or, as expressed in anotherway, in accordance with the demands upon the heating system,subcalibration of the thermostat is greatly avoided and a substantiallyconstant indoor temperature obtained.

It is obvious that by the use of my invention the frequency of operationof the thermostat'2l may be varied within wide limits depending upon thechoice of value of resistance 34 and 36, and that the temperature withinspace III may be maintained at a substantially constantvalue over wideranges of operation.-

It will furthermore be obvious to those skilled in the art that myinvention is not restricted to the embodiment disclosed in the drawingbut that it is applicable to all heating systems whether they be of thetype in which the position of a damper or valve is regulated constantly,as by a floating control, or operated from one extreme to the other, asby the on-off-type of control.

What I claim as new and desire to secure by Letters Patent oi the UnitedStates is:

2. In combination, a thermostat, a heat ex- Y changer influencing saidthermostat and controlled thereby, a secondary source of heat comprisingan electric heater influencing said thermostat, and means afiected bycurrent flow through the secondary source of heat for varying the effectof said secondary sourceoi heat on said thermostat.

3. In combination, means for heating a space, an auxiliary electricheater and a space temperature responsive thermostat for placing both ofsaid heaters in operation upon a call for heat, the auxiliary'heaterbeing arranged to heat the thermostat locally, and means including ahigh temperature coeflicient resistance surrounded by material having ahigh thermal capacity and positioned so as not to affect said thermostatthermally in series with said auxiliary heater for varying the effect ofthe auxiliary heater on said thermostat.

4. In combination, means for-heating a space, an auxiliary electricheater, a space responsive thermostat for placing both of said heatersin operation on a call for heat, the auxiliary heater being arranged toheat the thermostat locally, and a high temperaturecoefllcientresistancein series with said auxiliary heater for decreasing the effect of theauxiliary heater on said thermostat in response to increased demands onsaid heating means. a

locally, and means aflected by current flow' through the auxiliarytemperature changer for 2,156,089 additionally controlling the eflect oithe auxiliary temperature changer on said thermostat.

'6. In a temperature control system, the combination including a maintemperature changer for changing the temperature of a space to becontrolled, a space temperature responsive thermostat, thermo-calibratedto operate to a plurality of control positions at predeterminedtemperatures in control of the temperature changer, an auxiliarytemperature changer also controlled by the space temperature responsivethermostat adapted to affect said thermostat locally, said auxiliarytemperature changer having a heating capacity sufflcient to operate saidthermostat to one of its control positions in a brief interval forintermittently placing said main temperature changer into operation, andmeans affected by current flow through the auxiliary temperature changerfor decreasing the eifect of said auxiliary heater on .said thermostatin response to increased demands on said main temperature changer forpreventing subcalibration of said space temperature responsivethermostat.

7. In combination, an enclosure, a thermostat, temperature changingmeans influencing said, thermostat and controlledthereby, an auxiliarytemperature changing means having a predetermined time constantinfluencing said thermostat and having the time of operation thereofcontrolled by said thermostat, and thermal timing means within saidenclosure connected with said auxiliary temperature changing means andhaving a greater time constant than the auxiliary temperature changingmeans for varying the effect of the latter in proportion to thepercentage of time said auxiliary temperature changing means is inoperation.

8. In combination, temperature control means including a thermostatresponsive to variations in the temperature to be controlled and havinga preheater controlled by said thermostat for locally heating saidthermostat only during predetermined temperature variations, andseparate timing means operable with a time delay under the control 01'said thermostat for varying the heating efiect 01' said preheater towarda pre determined minimum limit during the time said predeterminedtemperature variations occur and toward a predetermined maximum limitduring other times.

HARRY R. CRAGO.

